The present invention relates to a transmission device for motorcycles, two wheeled vehicles and other vehicles and more particularly to a transmission device wherein a centrifugal clutch is mounted on a crank shaft of the engine. A multiple plate type speed change clutch is mounted on a speed change input shaft, and an output member of the centrifugal clutch is connected to an input member of the speed change clutch through a reduction gear train.
In a transmission device of the foregoing species, the centrifugal clutch is in its half-set state for a relatively long period of time in order that the vehicle may be started smoothly. As a result, a large amount of frictional heat is generated, which requires that the clutch must always be cooled. On the other hand, the speed change clutch is in its half-set state for a very short period of time, because the speed change clutch is momentarily opened and closed when the speed change gear is switched. However, a turning torque of the crank shaft increased by the reduction gear train, must be positively transmitted to the speed change input shaft through a frictional connecting portion without any substantial slippage.